region: Don't use gtk_widget_show_all

It is removed in GTK+ 4
11 jobs for region-no-no-show-all in 3 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #122821
build

00:01:57

 
  Test
passed #122823
asan

00:01:36

passed #122822
test

00:01:49

 
  Manual
manual #122828
allowed to fail manual
armv8
manual #122824
allowed to fail manual
flatpak
manual #122827
allowed to fail manual
msan
manual #122829
allowed to fail manual
ppc64le
manual #122825
allowed to fail manual
tsan
manual #122826
allowed to fail manual
ubsan
 
  Review
manual #122830
allowed to fail manual
review
manual #122831
allowed to fail manual
stop_review